1. Field of the Invention
The present invention relates to a data transmission apparatus suitable for use in for example a near-video-on-demand (NVOD) system able to deliver audio and video (AV) data upon request and to an audio and video data server provided with a plurality of the same.
2. Description of the Related Art
Realization of a video-on-demand (VOD) system which can instantaneously provide program data in response to requests from receivers has long been desired. A VOD system which can respond to individual requests made at any time, however, would require a data transmission apparatus which could handle extremely large scale, complicated processing.
Therefore, an NVOD system has been proposed which can function similarly to a VOD system but requires a simpler configuration of the data transmission apparatus. The NVOD system transmits program data over a plurality of channels at predetermined time intervals and responds to requests made within certain time zones by selecting the channel of the program starting from the next time zone and sending that program.
In a system for transmitting program data like the NVOD system, however, it is sometimes not possible to provide the program data suitably in response to the request of a receiver. That is, when the time interval between the program data being transmitted in parallel is too long, one cannot really say that the receiver can view the program data almost instantaneously.
The time interval between program data being transmitted in parallel is in many cases determined by the number of channels which can be used for the transmission of a single program. For example, sometimes the time for reproduction of the program data divided by the number of channels is used as the time interval between program data and the program data is sent over the plurality of channels staggered equally by that time interval.
As a result of this, however, if trying to transmit a program with a relatively long reproduction time, the time interval becomes long and a problem arises in terms of the "instantaneousness" of the response. For example, when offering a two-hour long program, the program would start at 30 minute intervals in the case of use of four channels or 15 minute intervals in the case of use of eight channels, so a waiting time of as long as 30 minutes or 15 minutes would be required.
This means that if trying to shorten the maximum waiting time, it is necessary to increase the number of channels used for a single program. For example, if trying to keep the maximum waiting time in a program of about two hours length mentioned earlier to 5 minutes, use of 24 channels would be required. If using such a large number of channels for a single program, the problem would arise of the number of types of the programs offered as a whole becoming smaller.